Biomedical research on the European species of mistletoe, Viscum album, clearly indicates this plant produces biologically active compounds (probably lectins) which possess useful, anti-carcinogenic activity. A plant species from the same family, American mistletoe (Phoradendron serotinum) is a parasite of the crowns of many species of tress in the eastern United States. American mistletoe has not been studied for the presence of any biological activity of it secondary metabolites (lectins, flavonoids, alkaloids). Our pilot experiments with P. serotinum have shown that 15 fractions from a polyamide column have potent insect anti- feedant, anti-microbial, and allelopathic activity. Given this evidence, we believe P. serotinum should be investigated for the possible anti-tumor activity of its lectins and flavonoids. Bioassays will be used to determine the efficacy of these plant natural products as anti-cancer patients. One antimicrobial assay and three in vitro assays will be done using a human leukemia cell line, a human melanoma cell line, and a large cell lung cancer line. The fourth assay will involve Agrobacterium tumefaciens which contains tumor-inducing plasmids. This assay has been shown to have a significant correlation with the ability of plant extracts to reduce the rate of development of murine leukemia. Our model predicts that the production of secondary metabolites in American mistletoe- their quantity, structure, and potency- is environmentally induced by factors including species and condition of host tree, and by recent history of pathogen and herbivore challenge to mistletoe. Lack of this type of ecological insight may partially explain the variability in medicinal quality of plant natural products, especially for species of mistletoe. American mistletoe, Phoradendron serotinum, is an excellent candidate in the search for biologically active compounds including new anti-cancer agents, and merits the same careful study now being given to its European counterpart Viscum album.